1. Field of the Invention
This patent application pertains to an apparatus for testing blood constituents. More particularly, this application pertains to such an apparatus utilizing spectrophotometric analysis of blood constituents.
2. Description of the Prior Art
The use of spectrophotometric methods to quantitatively determine the concentration of a blood constituent are known. For example, U.S. Pat. No. 4,882,492 to Schlager teaches a non-invasive near-infrared measurement of blood analyte concentrations. The Schlager patent is particularly directed to the measurement of blood glucose levels. The Schlager patent recognizes that certain wavelengths of light in the near-infrared spectrum are absorbed by glucose. Modulated light is directed against a tissue (shown as an earlobe). The light is either passed through the tissue or impinged on a skin surface. The light is spectrally modified in response to the amount of analyte (for example, glucose) in the blood and tissue. The spectrally modified light is split with one beam passed through a correlation cell. The other beam is passed through a reference cell. The intensity of the beams passing through the correlation cell and the reference cell are compared to calculate a glucose concentration in the sample.
U.S. Pat. No. 4,805,623 to Jobsis teaches a spectral photometric method for quantitatively determining the concentration of a component in human blood. The Jobsis method teaches various steps including the determination of an apparent effective path length for the light which is being absorbed by the constituent being measured.
U.S. Pat. No. 4,655,225 to Dahne et al. teaches a spectrophotometric method and apparatus for non-invasive testing. The Dahne patent is particularly directed to the measurement of blood glucose.
U.S. Pat. Nos. 4,014,321 and 3,958,560 to March teach non-invasive glucose sensor systems which involve passing light through the cornea of the patient.
U.S. Pat. No. 4,427,889 to Muller, dated Jan. 24, 1984, teaches a method and apparatus for molecular spectroscopy. In the embodiment described within the patent, blood glucose is determined through absorption analysis of infrared wavelengths absorbed in a glucose-containing sample (such as whole blood or urine).
Notwithstanding the developments in the art, a need for an improved spectrophotometric measurement apparatus and method persists. For example, systems and methods which require the calculation of an apparent light pathway are susceptible to inaccuracy. Such a system is shown in the aforementioned U.S. Pat. No. 4,805,623. Systems which have fixed dimensioned light pathways (for example, U.S. Pat. No. 4,014,321) are restricted in their use and practicality. It is also desirable to develop a system and apparatus which can be used for non-invasive testing as well as invasive testing (for example, as a continuous monitor for testing blood glucose level during surgery or insulin treatment). Further, it is desirable to develop a system which can be used in conjunction with a chemical emission system (such as a blood glucose monitoring system which controls an insulin administering apparatus).
In addition to the foregoing, it is desirable to provide an apparatus which can perform multiple wavelength analysis (i.e., broad spectrum analysis) of a sample. A common technique for providing a broad spectrum analysis is a so-called Fourier transform infrared ("FTIR") spectrometer. Such spectrometers are well known in the art and include means for generating a beam of a source light. The beam is then split by a beam splitter into a reference light and a test light. The reference light is reflected off of a fixed mirror to a light detector. The test light is passed through or reflected from a sample and off a moving mirror. The moving mirror translates the information from the frequency domain to the time domain to produce an interferogram. When passed through the sample, various frequencies of the light are absorbed at various rates depending upon the constituents of the sample. The light from the reflected test beam is directed to the light detector where it crosses paths with the reference beam. The modified test beam and the reference beam interact to form an interference pattern which is detected by the light detector and processed through a processor to generate a signal indicative of the constituents in the sample.
The use of Fourier transformed infrared spectrometry (FTIR) in testing for blood constituents (such as blood glucose) is known. A description of such a test is described in a paper entitled "Determination of Physiological Levels of Glucose in an Aqueous Matrix with Digitally Filtered Fourier Transform Near-Infrared Spectra" by Mark A. Arnold and Garry W. Small of the Department of Chemistry, University of Iowa, Iowa City, Iowa, as published in Analytical Chemistry, Vol. 62, No. 14, Jul. 15, 1990.
In prior FTIR analysis, the test sample (such as a volume of blood) would be removed from a patient and placed on a FTIR spectrometer. As a result, the test was invasive as well as non-continuous.
It is an object of the present invention to provide a technique for using FTIR spectrometry to test for blood constituents in a manner which can be both continuous and non-invasive.
III.